One of the recurring subjects I have covered on this blog is the contention that cranberry supplements can help prevent or treat urinary tract infections. Each time I address the subject, the evidence points to pretty much the same general conclusion:

There is weak theoretical justification for using cranberry products for UTIs, though none of the supporting preclinical evidence involves dogs or cats. There is conflicting clinical trial evidence in humans, and no clinical studies in dogs and cats.

This [Cochrane] review indicates pretty clearly that overall, cranberry juice is not effective in preventing UTIs despite theoretical reasons why it might be. This illustrates, yet again, why we cannot rely on extrapolation from pre-clinical or in vitro studies to tell us what will work in actual patients.

The in vitro portion of this study is consistent with existing research that suggests cranberry extracts may reduce the ability of some bacteria to stick to the lining of the urinary tract. This could theoretically help prevent some urinary tract infections, though clinical research in human patients suggests this doesn’t really work to a significant extent in living people.

The portion of the study looking at prevention of UTIs in actual dogs, unfortunately, doesn’t help establish what benefit, if any, this product might have.

So basically, while there is a plausible argument that cranberry supplements might be helpful based on lab research, clinical studies don’t seem to show they actually do much in real human patients, and there isn’t any real research in veterinary patients. Fortunately, the last part of that conclusion is now less applicable, thanks to a newly published clinical trial in dogs. Unfortunately, the results of the trial tend to support the existing human literature which has failed to find any real-world benefits.

This study specifically evaluated cranberry extract as a method for preventing bacterial urinary tract infections in dogs with newly acquired neurologic dysfunction due to herniated intervertebral disks damaging their spinal cords. Dogs with this condition are predisposed to such infections, and it was hoped the supplement would reduce this risk. The supplement used, Crananadin, was the same I reviewed in my original post on this subject.

The study employed excellent methodology, with appropriate randomization, blinding, placebo control, and pre-determined objective outcome measures. The results were clear and consistent. The cranberry group actually had a higher rate of infections than the placebo group, though the difference was not statistically significant. And though pre-clinical evidence suggests cranberry supplements might be most likely to be useful with E. coli infections, there was no statistical difference between the groups in the rate of this type of infection, with most occurring in the cranberry group. The study was ultimately smaller than the investigators initially intended because an interim analysis clearly showed no trend towards any benefit and very little chance that a benefit would be seen even if more animals were enrolled.

While no single study should be the final word on any complex medical question, this trial is about as strong a clear a negative finding as one can get. And given the similar failure to find significant benefits in many human trials, it adds to a growing body of evidence suggesting that cranberry supplements are not useful in preventing or treating urinary tract infections. While additional studies may help clarify the issue, especially those involving different types of supplement and different patient populations, at some point consistent failure to find a benefit has to be seen as a reason to question the wisdom of expending scarce research resources on studying the question. We aren’t, in my view, quite at that point yet, but at least clinicians should be clear with pet owners that the evidence does not look good for cranberry supplements, and these should not be relied on in lieu of effective monitoring and medical treatment.

Bottom Line
Despite some promising laboratory studies suggesting cranberry supplements might help prevent or treat urinary tract infections, the evidence of studies in clinical patients has been disappointing. Conflicting studies in humans suggest, on balance, that there is probably no significant benefit. And now a high-quality clinical trial in dogs has failed to find any effect, even in the the of infections the pre-clinical research most strongly suggested there should be one.

While the risks of cranberry supplements are probably negligible, pet owners should understand, and veterinarians should make in clear to their clients, that there is no good reason to believe they have any real value in preventing or treating urinary tract infections.

The in vitro portion of this study is consistent with existing research that suggests cranberry extracts may reduce the ability of some bacteria to stick to the lining of the urinary tract. This could theoretically help prevent some urinary tract infections, though clinical research in human patients suggests this doesn’t really work to a significant extent in living people.

The portion of the study looking at prevention of UTIs in actual dogs, unfortunately, doesn’t help establish what benefit, if any, this product might have. No infections occurred in any of the dogs regardless of the treatment they got, and the comparison was between 6months of daily use of the cranberry product and a 2-week course of an antibiotic, which seems unlikely to effectively prevent UTIs for 6 months. This raises questions about whether either of the treatments were really responsible for the lack of UTIs in the dogs or if this was the result of the Hawthorne effect or some other source of error.

Max, that study had an amazingly small sample size (6) and no placebo control. Was it blinded? It’s a completely useless experimental design if you want to know if the extract prevents UTIs. You need to have a large enough sample size and a long enough trial that your control group gets plenty of UTIs. That gives you a baseline without which you have nothing to compare against.